Knowledge of the rate of development of immature cardiomyocytes after implantation into a host heart is important for studies using cell therapy. To assess this functionally, we have implanted rat neonatal cardiomyocytes (NCMs) in normal and infarcted rat heart and re-isolated them for functional assessment. Maturation of implanted bone marrow stromal cells (BMSCs) was compared under similar conditions. NCMs from green fluorescent protein (GFP) transgenic rats were implanted into adult normal or infarcted rat hearts and re-isolated after 1, 2, or 4 weeks by standard enzymatic digestion. BMSCs labeled with DiI and iron oxide were implanted into rats with myocardial infarction and cells re-isolated 1, 2, 5, 6, and 16 weeks later. GFP-labeled myocytes approaching the adult morphology were detected 2 weeks after implantation of NCMs, but were significantly shorter than adult host myocytes and had reduced contractility. By 4 weeks after implantation, re-isolated GFP-labeled myocytes were close to the adult phenotype in contractile characteristics, although still significantly shorter. Infarction of the host did not alter the rate of maturation of implanted cells. After implantation of BMSCs, small numbers of functional DiI-labeled myocytes were re-isolated from 4/11 animals but were more mature than expected from the NCM studies. This adds evidence that BMSC-derived cardiomyocytes were not a result of transdifferentiation. The maturation rate of implanted NCMs represents a benchmark against which to evaluate the likely rate of formation of fully functional cardiomyocytes from implanted cells.